Plasmon dispersion of the Au(111) surface with and without self-assembled monolayers

Polygonal gold nanocrystal induced efficient phase transition in 2D-MoS2for enhancing photo-electrocatalytic hydrogen generation

Plasmonic nanocrystals (NCs) assisted phase transition of two-dimensional molybdenum disulfide (2D-MoS₂) unlashes numerous opportunities in the fields of energy harvesting via electrocatalysis and photoelectrocatalysis by enhancing electronic conductivity, increasing catalytic active sites, lowering Gibbs free energy for hydrogen adsorption and desorption, etc. Here, we report the synthesis of faceted gold pentagonal bi-pyramidal (Au-PBP) nanocrystals (NC) for efficient plasmon-induced phase transition (from 2 H to 1 T phase) in chemical vapor deposited 2D-MoS₂. The as-developed Au-PBP NC with the increased number of corners and edges showed an enhanced multi-modal plasmonic effect under light irradiations. The overpotential of hydrogen evolution reaction (HER) was reduced by 61 mV, whereas the Tafel slope decreased by 23.7 mV/dec on photoexcitation of the Au-PBP@MoS₂hybrid catalyst. The enhanced performance can be attributed to the light-induced 2H to 1 T phase transition of 2D-MoS₂, increased active sites, reduced Gibbs free energy, efficient charge separation, change in surface potential, and improved electrical conductivity of 2D-MoS₂film. From density functional theory (DFT) calculations, we obtain a significant change in the electronic properties of 2D-MoS₂(i.e. work function, surface chemical potential, and the density of states), which was primarily due to the plasmonic interactions and exchange-interactions between the Au-PBP nanocrystals and monolayer 2D-MoS₂, thereby enhancing the phase transition and improving the surface properties. This work would lay out finding assorted routes to explore more complex nanocrystals-based multipolar plasmonic NC to escalate the HER activity of 2D-MoS₂and other 2D transition metal dichalcogenides.

Influence of N-introduction in pentacene on the electronic structure and excited electronic states

N-Heteropolycyclic aromatic compounds are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of 6,7,12,13-tetraazapentacene (TAP) adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer TAP adsorbs in a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) as well as the triplet state (T) are assigned. The optical gap (S₀ → S₁ transition) is found to be 1.6 eV and the T₁ energy 1.2 eV. In addition, thorough comparison to previously studied pentacene (PEN) and 6,13-diazapentacene (6,13-DAP) is made explaining in detail the influence of nitrogen substitution on the electronic structure and in particular on the intensity of the α-band in the UV/vis absorption spectrum. In the series PEN, 6,13-DAP, and TAP, the α-band (S₀ → S₂ transition) gains significantly in intensity due to individual effects of the introduced nitrogen atoms on the orbital energies.

On the fate of high-resolution electron energy loss spectroscopy (HREELS), a versatile probe to detect surface excitations: will the Phoenix rise again?

From its advent, high-resolution electron energy loss spectroscopy (HREELS) has emerged as one of the most versatile tools in surface science. In the last few decades, HREELS was widely used for the fundamental study of (i) chemical reactions at the surfaces of model catalysts (mostly single crystals), (ii) lattice dynamics (phonons), (iii) surface plasmons and (iv) magnons. However, HREELS has experienced a continuous decay of the number of daily users worldwide so far, due to several factors. However, the rise of Dirac materials (graphene, topological insulators, Dirac semimetals) offers new perspectives for HREELS, due to its unique features enabling ultrasensitive detection of (i) chemical modifications at their surfaces, (ii) Kohn anomalies arising from electron-phonon coupling and (iii) novel plasmonic excitations associated to Dirac-cone fermions, as well as their eventual mutual interplay with other plasmon resonances related to topologically trivial electronic states. By selected case-study examples, here we show that HREELS can uniquely probe these phenomena in Dirac materials, thus validating its outstanding relevance and its irreplaceability in contemporary solid-state physics, thus paving the way for a renewed interest. In addition, recent technological upgrades enable the combination of HREELS as an add-on to photoemission apparatuses for parallel readout of energy and momentum of surface excitations. Open issues for theoretical modelling of HREELS related to the dependence on primary electron beam energy and scattering geometry are also critically presented.

Anisotropic dispersion and partial localization of acoustic surface plasmons on an atomically stepped surface: Au(788)

Understanding acoustic surface plasmons (ASPs) in the presence of nanosized gratings is necessary for the development of future devices that couple light with ASPs. We show here by experiment and theory that two ASPs exist on Au(788), a vicinal surface with an ordered array of monoatomic steps. The ASPs propagate across the steps as long as their wavelength exceeds the terrace width, thereafter becoming localized. Our investigation identifies, for the first time, ASPs coupled with intersubband transitions involving multiple surface-state subbands.

Interplay between single-particle and plasmonic excitations in the electronic response of thin Ag films

High-resolution electron energy loss spectroscopy is used to study the electronic properties of thin Ag layers on Ni(111). In addition to the ordinary surface plasmon at 3.8 eV, we observe a broad feature at 7-8 eV, whose nature is investigated as a function of scattering geometry and primary electron beam energy. Loss measurements unambiguously indicate that this mode has spectral components from both free-electron Ag plasmonic excitations (free-electron surface plasmons and multipole plasmons) and single-particle transitions.

Correlated motion of electrons on the Au(111) surface: anomalous acoustic surface-plasmon dispersion and single-particle excitations

The linear dispersion of the low-dimensional acoustic surface plasmon (ASP) opens perspectives in energy conversion, transport, and confinement far below optical frequencies. Although the ASP exists in a wide class of materials, ranging from metal surfaces and ultrathin films to graphene and topological insulators, its properties are still largely unexplored. Taking Au(111) as a model system, our combined experimental and theoretical study revealed an intriguing interplay between collective and single particle excitations, causing the ASP associated with the Shockley surface state to be embedded within the intraband transitions without losing its sharp character and linear dispersion.

Vacuum phonon tunneling

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

Acoustic plasmon on the Au(111) surface

We report an acoustic surface plasmon mode on the Au(111) surface, which disperses into the visible region, as measured by high resolution electron energy loss spectroscopy. The new mode is assigned to an acoustic surface plasmon arising from the Shockley-type surface state electrons and coexists with the conventional surface plasmon. This low energy collective excitation disperses linearly up to ∼2.2  eV, i.e., into the visible region. The divergence from theoretical prediction appears to emphasize the importance of band structure effects upon the dielectric function of the surface region.